@Fotiman: You convinced me.
The HTML <b> tag is used for specifying bold text.
The intention with this tag is to markup text as bold without conveying any extra importance. For example, this could be useful in article abstracts, where the beginning of an article is set in bold text.
According to the HTML5 specification, this tag should be used as a last resort when no other tag is more appropriate. In particular, headers should use the <h1> to <h6> tags, stress emphasis should use the <em> tag, importance should be denoted with the <strong> element, and text marked or highlighted should use the <mark> tag.
You can also use the CSS 'font-weight' property to set bold text.
Therefore, from a SEO point of view, strong seems the right choice.